In the past, traditional drainage systems (such as for foundation drains, septic drainage fields, and other applications) have employed gravel aggregate and, optionally, a perforated pipe or pipes surrounded by the gravel aggregate. The gravel provides structure and void volume to the drainage system.
More recently, pre-assembled drainage units utilizing a geosynthetic aggregate surrounded by a perforated sleeve have been utilized instead of gravel aggregate. U.S. Pat. No. 5,015,123 shows examples of pre-assembled drainage units. One type described includes a length of perforated distribution pipe surrounded by a volume of loose lightweight pieces of plastic aggregate bounded by a perforated sleeve. Another type does not include the pipe, just the plastic aggregate bounded by the perforated sleeve.
U.S. Pat. No. 6,467,996 describes improved geosynthetic drainage beads for use as an aggregate in pre-assembled drainage units. The drainage bead described in this patent includes a central portion with several legs and ribs extending therefrom, the legs defining channels therebetween. The legs and ribs of the drainage bead help to maintain void volume in the drainage unit. This patent emphasizes that the legs should be wider than the channels, to prevent the beads from interlocking with one another. Additionally, this patent emphasizes that the central portion of the bead should be relatively thick, in order to maximize strength.
As with the gravel aggregate, the geosynthetic aggregates described in these two patents provide structure and void volume for the drainage systems.
Current practices limit the ability of the manufacturer to maximize the specific gravity and void volume of the drainage beads at a specified bulk density. Specific gravity may be used as one indicator of the compressive strength of the drainage beads (e.g. resistance to the compressive forces typically encountered in drainage systems), whereas void volume may be used as one indicator of performance of the drainage system (i.e. fluid holding capacity). Current practices use bulk density as the primary mechanism for controlling specific gravity. Raising bulk density, however, typically requires increasing manufacturing cost, since higher bulk density means a greater amount of raw materials will be required to produce the drainage unit.